Marco Simoncelli was fastest of the four 250cc stars making their MotoGP debuts during Monday's Valencia post-race test.

The 2008 250cc world champion took a podium finish with the 1000cc Aprilia RSV4 earlier this season in World Superbike, but still found the Gresini-run RC212V MotoGP machine to be an exceptional beast.

The Italian, who crashed out while leading Sunday's 250GP title decider, confirming Hiroshi Aoyama as champion, completed 51 laps on his way to 14th position on day one. Simoncelli's best time was 2.569sec from the top but 0.543sec clear of former quarter-litre rival Alvaro Bautista (Suzuki).

"What a difference!" said Simoncelli. "It is not uncontrollable but the engine pulls a bit! My first impression is that these bikes are not as easy as people say and like with anything new you have to work hard at it. I have given my thoughts to the engineers and I'm sure I'll be faster tomorrow.

"We started out with a setting designed for a rider with completely different physical dimensions to me and quickly made progress with the changes we made to adapt it. We need to keep working like this and not try to get ahead of ourselves in this test. I just have to get an understanding of the bike and find a direction with the set-up," he added.

Of the two other 250cc graduates, Hector Barbera (Ducati) was 16th and Aoyama (Honda) 17th.

On the other side of the Gresini garage, Monday's test also saw Marco Melandri return to Gresini Honda - with whom he finished second in the 2005 world championship - after an absence of two years.

Reunited with crew chief Antonio Jimenez, the former Ducati and Hayate rider was 1.179sec from Casey Stoner, but a solid 0.624sec under his best ZX-RR race lap for tenth on the timesheets.

"It was a nice day, first of all going into the garage today and then getting on the bike," said Marco. "It was a nice surprise to see how much the 800cc Honda has changed since 2007 - it is a very different bike to the one I rode that year. I like it and we quickly got to work on the geometry, finding a good base to work from.

"Tomorrow we'll turn our attention to the suspension and a couple of other little things but for now I am happy. The engine works differently to the Kawasaki so I need to get my feeling for the throttle back but my pace was consistent today, not far behind the other Hondas, and we got an impression of how much room there is for improvement. Not bad, not bad!" he smiled.

"This was basically the first day of the new season, with some important debuts being made," declared team manager Fausto Gresini. "Marco Simoncelli made his first appearance in MotoGP on a Honda and Marco Melandri rejoined us to complete the all-Italian dream team.

"Simoncelli has started out carefully, in the right way, trying to understand a bike, brakes and tyres that are totally different to anything he has ridden before. The system of working is also totally different for him and the mechanics but the first day went well.

"Melandri was in enthusiastic, positive and determined mood. It looked liked he had fun, his lap times were good and there is room for more. All in all in MotoGP it was a positive day and we're all delighted.

"It was also the first day of Moto2 testing for us with the Moriwaki test rider Simeon, a Belgian youngster who has just won the Superstock Championship. Simeon is only 20 but we decided to give him the opportunity to ride our bike while we make our final decision on who our riders will be in 2010," Gresini added.